Three solutions for you Oliver; quality eq , lower its crossover point, use RCF principles from the ESW1015 (or the original ESW1018 concept if you like but both will need extra volume), or even better use a combination and that's beside the fact that the peak will be less if every second cab is put upside down, and therefore the cube form comes in handy .

Don’t let the fun turn into frustration, it’s not worth. Just look to what you have accomplished so far (no, didn’t mean that sarcastic this time). You seem to dig into all details just because you cannot accept data without understanding, nothing wrong with that. Maybe it’s just time to build something, even if it is made from cheap chipboard. Just get the adrenaline rush from your first build: cutting wood, smelling glue & dust and getting thrilled by its first sound (even it’s not perfect). It will empty your head!